1. Field of the Invention
The present invention relates to an optical disk drive which can reproduce signals recorded on an optical disk.
2. Description of the Prior Art
Recently, there has been a desire to increase the signal density of optical disks and various approaches have been proposed. The diameter "D" of an optical spot converged on the signal surface of an optical disk can be expressed generally as follows: EQU D=1.21.lambda./NA,
wherein .lambda. denotes the wavelength of a light source and NA denotes the aperture number of an objective lens. Therefore, a higher density can be attained by using a shorter wavelength light source consisting of a laser diode and a larger NA objective lens. At present, it is expected that the wavelength can be shortened down to 600 nm using a GaAs III-V compound, but it is difficult to shorten the wavelength further. The increase in NA also has problems such as the difficulty of the formation of the objective lens and errors which to occur due to the disk inclination and the defocusing.
A ring belt aperture has been proposed to increase the density as an approach for obtaining a higher NA. FIG. 1 shows an example of a prior art optical disk drive using a ring belt aperture, illustrated in NIKKEI ELECTRONICS (No. 528, 129 (1991)). In FIG. 1, a laser beam emitted from a light source 1 consisting of a laser diode is converted to a parallel beam, and transmitted through a beam splitter 3 and is shaded by a disk-like shading member 4 arranged near the optical axis and is converged on the signal plane 7 of an optical disk on the rear side of an optical disk substrate 6. The light reflected from the signal plane 7 is collimated by the objective lens 5, shaded again by the shading member 4 near the optical axis and reflected by the beam splitter 3 to reach another beam splitter 8. The light reflected by the beam splitter 8 is transmitted through an optical system 9 and is detected by a detector 10 which detects focus error signals and tracking error signals for the beam spot on the signal plane 7. On the other hand, the light transmitted through the beam splitter 8 is collimated by a converging lens 11, and is transmitted through a pin hole 12 to be received by a detector 13. The detected signal is amplified by an amplifier 14, and is converted to a reproduced signal by a signal processing circuit 15.
FIG. 2 shows a readout signal 17 detected by the detector 13 and a reproduced signal 18 in relation to signal marks 16a, 16b, 16c and 16d provided on the signal plane 7 of the optical disk. In the signal processing circuit 15, the readout signal 17 is compared with a detection level 17R, and determined to be 1 or 0 based on whether or not the readout signal 17 exceeds a detection level, and then the reproduced signal 18 is obtained.
FIG. 3(a) displays an effect of the insertion of the shading member 4 wherein the ordinate represents the light intensity and the abscissa represents the distance from the optical axis in the lateral directions (.epsilon.- and .eta.-axes). When the shading member 4 is inserted as shown in FIG. 1, the light just after the transmission of the aperture plane of the objective lens 5 has a shape of a ring belt. The light intensity distribution 19b on the focal plane (signal plane 7), shown in FIG. 3(a), is calculated for an NA=0.45-0.70 ring belt aperture and a 0.78 .mu.m of wavelength. A light intensity distribution 19a with a circular aperture (NA=0.54) on the focal plane is shown for comparison. Because the aperture area is chosen to be the same, if the amount of light of the two cases are same, then the Strehl intensity (peak intensity) are the same. The distribution 19b for ring belt aperture has a smaller diameter of main lobe than the distribution 19a for circular aperture, while the former has a disadvantage that the side lobe rises. Therefore, if there are signal marks at the side lobe position, the effect thereof becomes large, so that cross talk and interference between signals arise strongly in the readout signals.
FIG. 3(b) illustrates an effect of the insertion of the pin hole 12. The optical intensity distribution 19c on the pin hole 12 shown in FIG. 3(b) corresponds to the light intensity distribution 19b for the ring belt aperture. Then, if the pin hole 12 allows only the main lobe of the converging light 19c to be transmitted through the pin hole 12, then the readout signal will not be affected by the signal marks located on the signal plane 7 on the side lobe positions of the conversion light 19b. Thus, cross talk and interference between signals can be decreased, and it is thought to be possible to reproduce high density signals.
Another effect of the insertion of the shading member 4 is that the focal depth becomes deeper. For example, a defocusing amount necessary to decrease the Strehl intensity by 20% is 0.70 .mu.m for a circular aperture of NA=0.70 while is 1.14 .mu.m for a ring belt aperture of NA=0.45-0.70 if the wavelength is 0.78 .mu.m. Therefore, by using a ring belt aperture, one of the disadvantages accompanying a large aperture, that is, a shallow focal depth and bad effects on defocusing, can be avoided.
However, the above-mentioned prior art optical disk drive has problems in the reproduction of high density signals. In order to decide if the reproduction of high density signals can be possible, a determination is made as to how the signal mark patterns shown in FIGS. 8(a)-(d) are reproduced.
FIG. 4(a) shows a diagram of observation waveforms (eye patterns) of the readout signal 17 for the patterns shown in FIGS. 8(a) and 8(b) obtained by theoretical calculation, while FIG. 4(b) is a diagram of observation waveforms (eye patterns) of the readout signal 17 for the patterns shown in FIGS. 8(c) and 8(d). Further, FIG. 4(c) displays an eye pattern wherein a defocusing by 1.14 .mu.m (of a degree such that the Strehl intensity of the beam spot decreases by 20%) is added on the reproduction of the patterns of FIGS. 8(a) and 8(b). An eye pattern is obtained as explained below with reference to FIGS. 10a-10c wherein the wavelength of the light source is 780 nm and the NA of the ring belt aperture is 0.45-0.70. The ordinate denotes a signal amplitude normalized by the detected light quantity (100) in a case that the signal plane 7 is a mirror plane, and the abscissa denotes the time corresponding to the scanning position of the beam spot. In FIG. 4(a) and 4(c), the diamond-like areas enclosing the x mark at the detection level (40), as represented by a reference sign "a", is called an eye. Because the 1 or 0 decision on the detection of signals is conducted by the comparison of the readout signal with the detection level, it is desirable that an eye is open both in the amplitude direction (in the vertical direction in the graphs) and in the time direction (in the horizontal direction in the graphs).
A first problem of the prior art optical disk drive is that cross talk and interferences between marks cannot be decreased by inserting the pin hole 12. It is confirmed from the results of the calculation that the existence of the pin hole 12 or the size of the pin hole 12 does not affect the eye patterns shown in FIGS. 4(a) and 4(b). (FIGS. 4(a) and 4(b) show the data without the pin hole 12.)
A second problem is that a large jitter "d" is generated due to the difference in pit patterns with adjacent tracks, as is clear from FIG. 4(a). Especially for the patterns of FIGS. 8(c) and 8(d), cross talk is very large, as shown in FIG. 4(b), and an eye 22A for the pit pattern shown in FIG. 8(c) and an eye 22B for the pit pattern shown in FIG. 8(d) are separated completely and an eye formed by the overlap of the eyes 22A and 22B is closed. That is, the signals cannot be reproduced.
A third problem is that as is clear from FIG. 4(c), an eye shifts largely upward by adding the defocusing, and jitter "d" at the detection level 40 is increased further. This tendency is common for other error factors as well as defocusing.
The generation of large jitter due to cross talk and defocusing makes it hard to reproduce high density signals by the prior art optical disk drive.